1. Field of the Invention
The present invention relates to method and apparatus for direct bonding of two bodies which may be different materials. More specifically, in the present invention, two bodies are directly and firmly bonded by means of hydrogen bonds between hydroxide groups adsorbed on the surface of one body and oxygen atoms on the surface of the other body. This invention is characterized in that the bonding can be conducted at room temperature under a low pressure without heating in a vacuum so that it is applicable to bonding between heterogeneous materials. The present invention may be applied to precise bonding between mechanically or thermally brittle functional parts such as micro sensors and IC chips.
2. Description of Related Art
Direct bonding of silicon wafers has been known, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2-46722, Japanese Examined Patent Publication (Kokoku) No. 62-27040, and Japanese Unexamined Patent Publication (Kokai) Nos. 62-122148 and 48-40372. FIGS. 1A to 1D illustrate a schematic model of the conventional bonding method. In this method, the surfaces of silicon wafers are preliminarily mirror polished, treated in a mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 solutions to make the surfaces of the wafers hydrophilic (to adsorb hydroxide groups), and brought into intimate or close contact with each other at room temperature while water is present between them, as shown in FIG. 1A. The wafers are then thermally treated to remove water between the wafers and bond the wafers through hydrogen covalent bonds, as shown in FIGS. 1B to 1D.
In this method, the thermal treatment requires a high temperature of above 300.degree. C., sometimes above 1000.degree. C. As a result, if this method is applied to bonding between materials having different thermal expansion coefficients, the bond between such materials is deteriorated due to the difference of the thermal expansion coefficient. Moreover, the hydroxide groups-adsorbing step is a wet process conducted in an aqueous solution and therefore cannot be carried out successively with a dry process conducted in a vacuum chamber.
The room temperature bonding of heterogeneous materials has been investigated using a process similar to the above one (leveling and affinity treatment in wet conditions), for example, that explained by J. Haisma and G. A. C. M. Spierings in "DIVERSITY AND INTERFACIAL PHENOMENA IN DIRECT BONDING", Proceedings of the 1. International Symposium on Semiconductor Wafer Bonding. Sci.-Tech. and Appl. (1992), p.18. This process relates to bonding between a mirror polished silica glass (SiO.sub.2) and materials deposited on a silicon wafer (Ta, Ti, Cu, W and so on). Heat treatment was not carried out since the thermal expansion coefficients of the materials to be bonded are different.
The effectiveness of this method however greatly depends on the materials. For example, tungsten cannot be bonded and copper can be bonded only under a substantial pressure. The reasons for these are not clear. Further, the reliability of the bonding is not high. This method therefore cannot be applied to bonding of bulks in practice.
The room temperature bonding of heterogeneous materials is also known in NIKKEI NEW MATERIALS, Sep. 14, 1992, pp.74-75. In this disclosed method, the surface of a material to be bonded is activated by applying a high speed beam of neutral atoms of an inert gas such as argon to remove an impurity layer from the surface of the material. The bonding is done by mounting a ceramic material onto the activated material and applying a pressure of about 10 to 20 MPa to the assembly. This method still has problems in industrial applications, since the pressure for bonding is as high as about 10 to 20 MPa and the presence of an amorphous layer 5 to 10 nm thick at the bonding interface causes a change in the electric properties at the bonding interface.